One thing they should know is that they always need to perform there own due diligence on these marketing companies. A lot of people can sound like they know what they are doing however many actually do not. I recommend reading neil patel or going onto quora and reddit and just read digital marketing group posts. Otherwise you can get played by someone that just says all the right things but brings you no real results at the end of the day. 

What we help our clients with is understanding the reasons behind our our choices. Which we do this with data. We can pull data on them and there competitors to explain why this choice will be made and also what they can even do to improve in the realm of digital marketing. We also use data from studies to make choices like with a facebook ad picture. If you use a very flashy picture with a sleek design and text it won't do as well as a facebook ad with a simple bold catchy picture and the necessary text. This was a study we found because of neil patel which it honestly makes sense. People are fast and want to know what you are selling prompty rather than having to put in work and read a whole bunch. 
Like in video ads you have 3 - 10 seconds to catch the viewers attention or else they are gone.